The present invention is situated in the domain of exhaust gas treatment systems installed in automotive vehicles. More precisely, the invention relates to a process for controlling the injection of urea in such a system, in order to optimize the operation of the treatment system. The process described here finds a particularly advantageous, but non-limiting, application in catalytic systems for selective catalytic reduction, called SCR, of nitrogen oxides emitted by an engine.
The principle of a SCR treatment system consists in chemically reducing the nitrogen oxides NOx by adding a reducing agent, such as ammonia (NH3) contained in urea or in any other reducing agent, for instance AdBlue, or DeNOXIUM, or C-Blue, upstream of a specific catalyst, called SCR catalyst. Such a system allows vehicles, in particular those equipped with diesel engines, to meet the legally permitted emission levels, which are continuously lowered.
Examples of nitrogen oxides are nitrogen monoxide, nitrogen dioxide, or any other chemical compound containing nitrogen and oxygen molecules.
In such a system, the urea injected in the exhaust line is converted to ammonia, in two successive chemical reactions:
first a pyrolysis reaction (NH2)2CO→HNCO+NH3, and then                a hydrolysis reaction HNCO+H2O→CO2+NH3.        
The ammonia thus obtained reacts in the SCR catalyst with the nitrogen oxides emitted by the engine. Depending on the composition of the exhaust flow, one or more of the following reactions can take place in the catalyst:                a so-called standard SCR reaction, reducing the nitrogen monoxides: 4NH3+4NO+O2→4N2+6H2O,        a so-called “fast SCR” reaction, reducing the nitrogen monoxides and the nitrogen dioxides: 4NH3+2NO+2NO2→4N2+6H2O. This reaction is certainly faster than the standard reaction, but requires equivalent quantities of nitrogen monoxide and nitrogen dioxide, and        a reducing reaction of nitrogen dioxide only, less fast than the previous reactions: 8NH3+6NO2→7N2+12H2O        
Given the conditions imposed by these chemical reactions, one of the main prerequisites for proper functioning of a selective catalytic reaction resides in the temperature of the exhaust gases. Indeed, this temperature must be sufficiently high:                at the urea injector, for proper decomposition of urea in ammonia, and        at the SCR catalyst, for proper catalysis of the nitrogen oxides.        
In the current state of technology there are numerous systems for treating nitrogen oxides using ammonia contained in liquid urea.
Such systems employ, for instance, urea injection strategies in which urea is injected only when the temperature in the exhaust line is greater than a predetermined value. Although effective during long distance driving, or in vehicles such as trucks, these systems, prove sometimes to be less efficient when installed in light vehicles driving in urban environment. Indeed, in these driving conditions, the temperature of the exhaust gas remains relatively low, because of the low speed and the frequent stops of the vehicle. Consequently, the injection of reducing agents is sometimes insufficient for correct treatment of the nitrogen oxides emitted by the engine.
In general, SCR catalysts used in treatment systems are built so that they can adsorb ammonia. When the temperature conditions do not allow the injection of urea, in numerous known systems it is possible to use the ammonia stored in the catalyst in order to start the chemical reactions for reduction of nitrogen oxide.
However, in such systems, it is difficult to control the quantity of ammonia stored in the catalyst. In this case, the catalyst is sometimes saturated with ammonia, which can lead to discharge of ammonia gas into the atmosphere. Since ammonia is an ill-smelling and irritating gas, such gas discharge is bothersome for the users of the vehicle.
Certain systems were also designed to heat the exhaust gases, continuously or during start-up, in order to ensure a sufficiently high temperature at all times. Nevertheless, these systems have in general the same previously described inconvenience, namely the inconvenience of regularly producing ammonia discharges into the atmosphere. Furthermore, the employed heating methods are energy intensive, which leads to relatively high consumption in the engine.